1. Field of the Invention
The present invention relates to a camera system using an interchangeable photographic lens, and in particular relates to a camera system adopting a dual motor control system for controlling an autofocus (AF) operation using a motor contained in the interchangeable photographic lens and a motor contained in the camera body. The present invention further relates an interchangeable lens of such a camera system.
2. Description of the Related Art
In conventional camera systems using an interchangeable photographic lens, an AF operation of the interchangeable lens is performed by a motor contained in the camera body (hereinafter referred to as an in-body motor) to which the interchangeable lens is mounted. However, in recent years, a camera system using an interchangeable lens wherein the interchangeable lens incorporates it's own motor (hereinafter referred to as an in-lens motor) and is configured to be capable of performing an AF operation using the in-lens motor, has been proposed.
For instance, technology for improving the operability of a digital camera system wherein a type of interchangeable lens which is driven by an in-lens motor and another type of interchangeable lens which is driven by an in-body motor are selectively mountable to the camera body has been proposed in Japanese unexamined patent publication 2006-259113. If the camera body is configured to be capable of performing an AF operation for each of various types of interchangeable lenses only with the in-body motor, the load on the in-body motor varies due to the difference between the specifications of the various types of interchangeable lenses, which sometimes causes difficulty in performing control of each interchangeable lens properly. However, providing each interchangeable lens with an in-lens motor in such a manner disclosed in the aforementioned Japanese unexamined patent publication to perform an AF operation makes it possible to achieve proper control of each interchangeable lens. Such a type of interchangeable lens that is provided therein with an in-lens motor is referred to as a dual-motor-compatible interchangeable lens in the present specification. In the camera system equipped with the dual-motor-compatible interchangeable lens, the camera body is configured to select, from among the in-lens motor and the in-body motor, the in-lens motor as the motor having priority use for an AF operation, upon the dual-motor-compatible interchangeable lens being mounted to the camera body.
In the dual-motor-compatible interchangeable lens, an ultrasonic motor that is small, lightweight and operates with a low noise level is adopted as the in-lens motor. This ultrasonic motor is composed of a stator having a plurality of circumferentially arranged piezoelectric elements that are polarized in the circumferential direction, and a rotor in the form of an annular plate or a circular plate, which is in press contact with the stator at a predetermined pressure. Applying a high-frequency voltage to the piezoelectric elements of the stator causes the piezoelectric elements to vibrate, and this vibration is amplified in the circumferential direction by comb teeth integral with the piezoelectric elements to thereby cause a progressive-wave motion of the comb teeth in the circumferential direction. As a result, the rotor which is in frictional contact with the piezoelectric elements rotates about the rotation axis of the rotor. In this ultrasonic motor, a predetermined contact pressure is continuously exerted on the abutment surfaces of the rotor and the stator by a resilient member to enhance the rotation efficiency of the ultrasonic motor (to enhance the rotational energy of the rotor with respect to the vibration energy of the stator).
Since the rotor is made to remain in press contact with the stator in this manner in the ultrasonic motor, the surfaces of the stator and the rotor which are in press contact with each other stick (adhere) to each other by an interfacial force produced therebetween to thereby cause an increase in static friction therebetween. As a consequence, a large torque may be required to rotate the rotor when the ultrasonic motor is actuated, the rotor may not be able to start rotating smoothly to thereby cause a reduction of the start-up performance of the motor, or the motor may become impossible to rotate in the worst scenario. Accordingly, in the type of dual-motor-compatible interchangeable lens in which an ultrasonic motor is adopted as the in-lens motor, or in the camera body to which this type of dual-motor-compatible interchangeable lens is mounted, if the in-lens motor is driven to perform an AF operation of the interchangeable lens during a photographing operation (image capturing), a delay in start-up of the in-lens motor may cause a delay in the AF operation or make the AF operation impossible to be performed; accordingly, it is sometimes the case that a picture cannot be taken in a correct in-focus state. This sort of problem also occurs in a dual-motor-compatible interchangeable lens that is configured to perform a zoom control operations using the in-lens motor.